Don't you still have the PDSA (People's Dispensary for Sick Animals) in the UK?
It was available when I lived there 20 or so years ago. They help people with financial issues around regular vet bills, for much lower fees or for free. I'm sure if you went along there on your own with her and explained how your Mum feels, they would at least look to see if it was anything serious that would not heal without intervention. Wear your oldest daggiest jeans and jumper and look really sad, I'm sure they'll help you out!